ASSESSMENT OF STEEL STRUCTURES WITH A DUAL SYSTEM OF MOMENT FRAME WITH CONCENTRICALLY BRACE INFLUENCING REPEATED EARTHQUAKES

In the seismic active zones, structures are likely to be subject to multiple earthquakes due to the complex stress interactions among the tectonic plates. A large earthquake is usually followed by numerous aftershocks within a short period. Although aftershocks are normally smaller in magnitude, their ground motion intensities are often large and have different energy contents, compared to the mainshock. These aftershocks have a strong potential to cause additional cumulative damages to structures that have been already damaged by the mainshock and threaten life safety even when only minor damage has remained from the mainshock. Seismic design codes provide a design process based on a "design earthquake" and do not take into account the effects of the aftershocks on this design. In the present study, by investigating the effect of seismic sequences on structures with different lateral resisting systems, the existing gap within the design guidelines and codes can be filled. In this regard, the performance of steel structures consisting of dual moment-resisting frames and concentrically braced systems was investigated in the seismic sequence. To this end, different types of steel structures with 3, 5, 7, 10, 15, and 20 stories were designed and analyzed based on Iranian codes with and without seismic sequences using Perform-3D software. The fragility curves from non-linear analysis at different performance levels under FEMA356 were obtained and the seismic vulnerabilities were evaluated. The results showed that the effect of earthquakes on higher structures was attenuated as a result of a decrease in the magnitude of the earthquake spectrum within longer periods, leading to a reduction in vulnerabilities. Moreover, the structures performed well at the performance level of Life Safety (LS), intended by the design codes; therefore, designs produced based on such levels in the earthquakes and seismic sequences near-field were assumed to be reliable. According to the results presented in this study, the second earthquake, whose PGA is more than the first earthquake, affects the structural response in the seismic sequence. The probability of the exceedance damage at the performance level (IO) in the far-field earthquakes and seismic sequences is more than that of the near-field earthquakes, while the probability of exceedance damage at the performance levels (LS) and (CP) in 10-, 15-, and 20-story structures in the near-field earthquakes and seismic sequences is higher than that of far-field earthquakes. On average, by applying the second earthquake to the first earthquake scenario with the intensity of 0.6g, the vulnerability of structures under near-fault and far-fault repeated earthquakes caused an increase in the single-earthquake ratio by 15\% and 20\% at the performance level of (LS), respectively.